The present invention relates to signal transmission for wireless mobile devices. More particularly, the present invention relates to a method and apparatus for reducing the bandwidth of a signal, while retaining the desired signal information and while satisfying system requirements, EVM requirements, and spectrum-mask requirements. The method and apparatus are applicable to both IQ-modulators and polar modulators, thereby allowing cost-effective polar modulators and/or cost-effective power amplifiers to be utilized. The method and apparatus are described below with reference to examples utilizing M-ary phase shift keying (M-PSK), such as quaternary phase-shift keying (QPSK) and 8-PSK; and M-ary quadrature amplitude modulation (M-QAM); however, the invention is not intended to be limited to such and may be applied to other approaches for signal generation.
The bandwidth in wireless digital communication systems is a limited resource. Accordingly, in most communication standards, such as GSM (Global System for Mobile Communications), EDGE (Enhanced Data Rates for GSM and TDMA/136 Evolution) and WCDMA (Wideband Code Division Multiple Access), the frequency bandwidth of a transmitted signal is strictly regulated by the system specifications. In addition, to provide for good reception of the transmitted signal, the shape of the transmitted signal in the time domain is also regulated in the system specifications. However, the standards often allow for some small spectrum leakage outside the desired frequency band and also for some signal distortions in the time domain to allow for cost-efficient and current-efficient transmitter architectures. These allowances make it possible to reduce production costs and prices of hand-held mobile devices.
The allowed signal distortions in the time domain are often measured in terms of error vector magnitude (EVM), that is, as a ratio between the square root of the power of an error vector and the square root of the transmitted mean power. FIG. 1A illustrates an example of a constellation of transmitted signal points 101 relative to reference signal points 102 in the IQ-plane for a QPSK system architecture. As illustrated in FIG. 1A, for example, for signals based upon QPSK, the error vector is a vector representing the difference between a transmitted signal vector yi and a reference signal vector Ri. As indicated by Equation 1, determining the EVM involves summing the squares of the magnitudes of a number of error vectors                     EVM        =                                                            1                N                            ⁢                                                ∑                                      i                    =                    1                                    N                                ⁢                                                                                                e                      i                                                                            2                                                                                    1                N                            ⁢                                                ∑                                      i                    =                    1                                    N                                ⁢                                                                                                y                      i                                                                            2                                                                                        (        1        )            In general, the magnitude of the EVM should be less than some prescribed value. For example, in WCDMA, the EVM should be less than 17.5%.
In addition, the allowed frequency distortions are often measured using a spectrum mask, an example of which is illustrated in FIG. 1B. FIG. 1B illustrates an example of an ideal (or reference) signal spectrum 103, a signal spectrum for a transmitted signal 104, and a spectrum mask 105 that defines the allowable extent of high-order frequencies for the transmitted signal spectrum. In other words, the spectrum mask 105 specifies the allowed shape of the frequency spectrum of the transmitted signal. Those skilled in the art will recognize that the signal spectrum for a transmitted signal 104 may extend beyond the bounds of an ideal signal spectrum 103 such as illustrated in FIG. 1B because the transmitted signal may possess high-order frequency components.
In many communication systems, linear modulation, such as QPSK (WCDMA) and 8-PSK (EDGE) are used. A common method for generating and transmitting linear modulation like QPSK over a bandwidth-limited wireless link utilizes IQ-modulation, such as illustrated in the block diagram of FIG. 2. FIG. 2 illustrates a conventional signal generation and transmission system 200 based upon IQ-modulation, comprising a symbol mapping unit (SM) 201, an up-sampling unit (↑N) 202, a pulse-shaping filter (h) 203, a digital-to-analog converter (D/A) 204, a low-pass filter (LPF) 205, mixers 206 and 207, a combiner 208, a power amplifier (not shown), and an antenna 209. The mixers 206 and 207 may be considered to form a modulator.
As illustrated in FIG. 2, data bits b(n) are mapped to symbols using the symbol mapping unit 201 (n refers to pulse numbering). The output from the symbol mapping unit 201 is a sequence of symbols, each of which can be represented as a complex quantity I(n)+jQ(n) with two components, I(n) and Q(n). Examples utilizing such mapping are QPSK and 8-PSK, which are well known in the art. It should be noted that the I and Q components retain their separate character throughout the signal processing in the system 200 until the I and Q components are summed and amplified in the power amplifier.
With reference to FIG. 2, the I and Q output signals from the symbol mapping 201 unit together form a complex-valued pulse train that is up-sampled in the up-sampling unit 202. The up-sampled pulse train is then fed into the pulse-shaping filter 203. The pulse-shaping filter 203 provides the desired shape in the time and frequency domains to the baseband signal according to the system specifications. The output signals from the pulse-shaping filter 203 are then digital-to-analog (D/A) converted in the D/A converter 204, and the resulting output signals are then fed into the low-pass filter 205. The I and Q components of the baseband signal are then fed into mixers 206 and 207, respectively, to generate I and Q radio frequency (RF) signals from the supplied baseband signals. The I and Q radio frequency signals are then combined and amplified in the power amplifier and are then directed to the antenna 208 for transmission over the air.
Conventional signal generation and transmission systems employing IQ-modulation have power amplifiers with stringent linearity requirements given the modulation depth (ratio between the maximum and minimum amplitude of a transmitted signal) for conventional I and Q signals. In conventional systems, these stringent linearity requirements necessitate the use of high-performance power amplifiers that are expensive.
Another method for generating linear modulation utilizes a polar modulator. Polar modulation operates on the principle that complex-valued symbols may be represented in polar coordinates (r, φ) corresponding to amplitude and phase instead of rectangular coordinates (I, Q). FIG. 3 illustrates a conventional signal generation and transmission system 300 based upon polar modulation, comprising a symbol mapping unit (SM) 301, an up-sampling unit (↑N) 302, a pulse-shaping filter (h) 303, a D/A converter (D/A) 304, a low-pass filter (LPF) 305, a rectangular-to-polar mapping unit 306, a phase modulator (PM) 307, an amplitude modulator 308, a power amplifier (not shown), and an antenna 309.
As illustrated in FIG. 3, data bits b(n) are mapped to symbols using the symbol mapping unit 301. The output from the symbol mapping unit 301 is a sequence of symbols, each of which can be represented as a complex quantity I(n)+jQ(n) with two components, I(n) and Q(n). The output signals from the symbol mapping unit 301 form a complex-valued pulse train and are up-sampled in the up-sampling unit 302. The output signals from the up-sampling unit 302 are then fed into the pulse-shaping filter 303. The I and Q signals output from the pulse-shaping filter 303 are then mapped to polar coordinates (phase and amplitude) in the mapping unit 306. The phase signal output from the mapping unit 306 is input to the phase modulator 307, which generates the phase portion of the radio signal. The amplitude signal output from the mapping unit is D/A converted in the digital-to-analog converter 304, the output from which is then fed into the low-pass filter 305. The amplitude signal output from the low-pass filter 305 and the phase signal output from phase modulator 307 are then combined in the amplitude modulator 308. The output signal from the amplitude modulator is then amplified by a power amplifier (not shown) and directed to the antenna 309.
An advantage of a system architecture utilizing a polar modulator is that the radio part of the system can be produced at lower cost compared to a conventional IQ-modulator. Another advantage is that a polar modulator consumes less current than a conventional IQ-modulator. However, there are drawbacks to utilizing a polar modulator. In particular, linear modulation is optimized for linear signal generation with conventional IQ-modulators, and the generation of linear modulation with a polar modulator results in amplitude and phase signals having a very high bandwidth. Accordingly, to prevent unwanted distortion of the output signal y(t) in a conventional polar modulator system, the phase modulator 307 and the amplitude modulator 308 must be produced with very high quality components such that they have sufficient bandwidth capability to effectively provide the high-bandwidth amplitude and phase signals. Typically, the phase modulator 307 and the amplitude modulator 308 must have a bandwidth capability about 3–5 times the bandwidth of the IQ signals in order to effectively represent the phase and amplitude signals without encountering unacceptable levels of distortion in the output signal y(t). Stated differently, the phase modulator 307 and the amplitude modulator 308 must have the above-noted bandwidth capability to fulfill the imposed system requirements such that the EVM does not exceed the prescribed level and such that the power spectrum of the transmitted signal does not exceed the limits of the spectrum mask. As a result, the required phase modulator 307 and amplitude modulator 308 are expensive, and the cost of these high-performance devices largely negates the cost savings and current-consumption savings associated with the radio part of a system utilizing a polar modulator.
Accordingly, there is a need for a method of signal generation that reduces the phase signal and amplitude signal bandwidths of a polar-modulation signal, and that reduces the modulation depth of an IQ-modulation signal, while retaining the desired signal information and while satisfying system specification requirements, EVM requirements and spectrum-mask requirements such that cost-efficient and current-efficient polar modulators and/or power amplifiers can be utilized. There is also a need for an apparatus that accomplishes these goals.